1. Field of the Invention
The present invention relates to an image forming device which has a motor capable of varying its driving load in accordance with temperature.
2. Description of the Related Art
In conventional various equipment having a motor for driving a load, such as an image forming device, the driving load increases due to thermal contraction of an object to be driven or increase in lubricant viscosity, in the case where such equipment is left at a low temperature.
In a device which is assumed to operate at a low temperature, therefore, a large motor enough to obtain a required torque, or a motor driven by a large current is used, so as to obtain the driving load required at the lowest temperature to ensure operation.
However, the torque, adjusted for the device to operate at a low temperature, is too high within a normal temperature range. This prevents the motor cost from being reduced, and the image forming device from being miniaturized.
In response to this, taking advantage of the relation that a motor torque becomes higher as an RPM (revolutions per minute) decreases as shown in FIG. 3, Japanese Unexamined Patent Application Publication No. 06-121592 proposes that a motor is driven at a low RPM so as to compensate for insufficient torque occurred at a low temperature in the case where an operating temperature of the motor is lower than the preset temperature.
However, in such a conventional device, where the operational speed of the motor varies simply with temperature, it is difficult for the device to form a steady image. One is because the device is not warmed up properly. Another is because the operational speed (RPM) of the motor changes at an inappropriate timing during image formation.